Ionomer resins are now in widespread use as the golf ball cover material. Ionomer resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, wherein some acidic groups are neutralized with metal ions such as sodium or zinc ions. Because of the excellent properties of resilience and scratch resistance, the ionomer resins are best suited as the base resin of the golf ball cover stock.
Since golf balls using ionomer cover stocks were marketed, cover stocks surpassing the properties of the ionomer cover stock have not been commercialized. The ionomer covers predominate in the current golf ball covers.
An attempt to develop a new type of cover stock surpassing the properties of the ionomer cover stock requires to find another resin that can be blended with an ionomer resin as the base so as to improve the properties of the blend without detracting from the properties of the ionomer resin itself.
JP-B 63-58856 discloses a cover stock in which a medium to low density polyethylene is blended with an ionomer resin as the base. More specifically, 1 to 9 parts by weight of a medium to low density polyethylene is blended with 100 parts by weight of an ionomer resin to formulate a cover stock. This is effective for improving the durability of a golf ball.
Of the medium and low density polyethylene resins used therein, the low density polyethylene polymerized by the high pressure process is characterized by a low hardness and low cost. It is thus expected that a low cost cover stock providing a soft feel when hit is obtained by blending the low density polyethylene with the ionomer resin. It has been desired to develop a cover stock by blending with the ionomer resin a more proportion of the high-pressure-produced low density polyethylene.
However, the high-pressure-produced low density polyethylene is less dispersible in the ionomer resin because it contains many branches owing to the polymerization process. For this reason, the high-pressure-produced low density polyethylene is not recommended in JP-B 63-58856.
Allegedly, if more than 9 parts by weight of a medium to low density polyethylene is blended with 100 parts by weight of an ionomer resin, the blend experiences a substantial loss of resilience to below the practical level because of the poor dispersion of the medium to low density polyethylene. Also allegedly, if a high density polyethylene is blended with an ionomer resin, despite the reduced content of polyethylene, the resulting cover stock loses durability and becomes impractical.